1. Field of the Invention
The present invention relates to the field of semiconductor manufacturing and, more particularly, to a memory module having series-connected printed circuit boards.
2. Description of the Related Art
In order to increase memory capacity, a memory module containing a plurality of memory chips on a single circuit board has been used. The memory module is inserted into a connector formed on a mother board. A plurality of connectors are provided on the mother board, thereby allowing expansion of memory capacity.
Since one connector receives one memory module, it is difficult to provide high-density and high-capacity packaging configurations and to provide high-speed performance due to the occurrence of branches. These problems are resolved by using a memory module comprising a plurality of printed circuit boards, each having memory chips mounted thereon. This technique has been disclosed in detail in U.S. Pat. No. 5,949,657 and is described below.
FIG. 1a is a cross-sectional view of a conventional memory module mounted on the mother board, and FIG. 1b is a perspective view of the memory module. FIG. 2 is a schematic view showing the connection of signal lines in the conventional memory module mounted on the mother board.
As shown in FIG. 1a, FIG. 1b, and FIG. 2, a memory module 100 comprises at least two printed circuit boards, i.e. a first circuit board 120a and a second circuit board 120b. Each of the first and the second printed circuit boards 120a, 120b comprises circuit wirings 121, a plurality of jumper pads 123, and conductive leads 125. A plurality of memory chips 111 are soldered to both sides of the first and the second printed circuit boards 120a, 120b and are electrically interconnected to the circuit wirings 121. The first and the second circuit board 120a, 120b are mechanically and electrically interconnected to each other via at least a flexible jumper assembly 130 connected to the jumper pads 123. The flexible jumper assembly 130 is formed in a xe2x80x9cUxe2x80x9d shape, and the first circuit board 120a and the second circuit board 120b are parallel-connected and are fixed to each other by a post pin 129. The jumper pad 123 of the first circuit board 120a is interconnected to the opposing jumper pad 123 of the second circuit board 120b via the flexible jumper assembly 130. The conductive leads 125 are inserted into the connector 141 of the mother board 140, thereby electrically connecting the memory module 100 to the mother board 140. In FIG. 2, reference numeral 150 is a control package, and reference numeral 153 identifies a signal line connecting individual memory modules 111 (see FIG. 1a).
In this conventional memory module, since two memory modules are inserted into one connector, the conventional memory module has the advantages of increasing memory capacity and of obtaining high mounting density.
However, in the conventional memory module, unit memory modules, i.e., the individual printed circuit boards, are electrically parallel-connected, thereby causing branches 132a and 132b in signal lines from the memory chips to the connectors. Thus, impedance mismatching increases, thereby also increasing the noise due to reflection. Therefore, it is difficult to reliably transmit data in a semiconductor memory device operated at high frequency.
In the case of using a memory module comprising memory chips having a speed of 100 MHz or more, for instance, a high-speed memory chip such as a double data rate synchronous DRAM (DDR) having a speed of up to 266 MHz, since the memory module is damaged by very small noise due to reflection at high-speed, it is difficult to transmit data reliably in the conventional memory module configuration. That is, the conventional memory module cannot be applied to high-speed chips.
Accordingly, an object of the present invention is to provide a memory module that prevents branches in the signal line of the memory and reduces reflection, thereby minimizing the occurrence of the noise due to reflection.
Another object of the present invention is to increase the memory capacity of a memory module by obtaining high mount density and high speed.
In order to achieve the foregoing and other objects, the present invention provides a memory module comprising a plurality of printed circuit boards, each comprising a plurality of memory chips mounted thereon and signal lines. Connection means electrically connects the printed circuit boards to each other. The connection means electrically series-connects the signal lines of each of the printed circuit boards.
Preferably, an end termination circuit is formed on a terminal of the signal line that is the most distant from a connector of a mother board.
Further, the memory chip preferably performs at a speed of approximately 150xcx9c300 MHz.
In one embodiment, the connection means is a flexible jumper assembly having a flexible film and a jumper line formed on the flexible film. Each of the printed circuit boards further comprises a jumper pad connected with the signal lines. The flexible jumper assembly interconnects the jumper pads of each of the printed circuit boards to each other. Among a plurality of the printed circuit boards, conductive leads are formed on an edge of one of the printed circuit boards. The conductive leads serve to connect the connection means to the circuit board.
The printed circuit board comprising the conductive leads is disposed on the first or the final of a plurality of the printed circuit boards.
The jumper pads of each of the printed circuit boards stand opposite to the corresponding jumper pads of other printed circuit board.